Advanced video compression systems are based on variable-size transforms to improve the compression factor. Each picture is segmented into picture blocks, the size of which is adapted to local picture contents. The segmentation must be transmitted to the receiver to allow it to apply a backward transform to the same blocks as those used by the encoder.
A known method of transmitting a segmentation map is disclosed in U.S. Pat. No. 5,241,395. Herein, the segmentation map is quadtree-encoded. A logic bit value `1` or `0` is assigned to each node of the quadtree-depending on whether or not the block which is associated with said node is further divided into smaller blocks. Small blocks have a plurality of higher level nodes and thus require many bits. Moreover, quadtree coding is limited to square blocks only.
An alternative method of transmitting video pictures based on a variable block-size segmentation is disclosed in European Patent application EP-A 0 220 706. Herein, the segmentation map is not separately transmitted. A special picture data codeword (ESC) identifies blocks which are divided into smaller blocks. The coded picture data for the smaller blocks are transmitted at a lower level.